SLOVYANSK, Ukraine — Ukrainian troops forced pro-Russian insurgents out of a key stronghold in the country’s embattled east Saturday, a significant success that suggested the government might finally be making gains in a months-long battle against a separatist insurgency.

As rebels fled from Slovyansk, vowing to regroup elsewhere and fight on, President Petro Poroshenko hailed the recapture of the city as “the start of a turning point” in a battle that has resulted in more than 400 deaths since April.

After a night of fighting that saw heavy artillery fire from Ukraine’s troops, government soldiers were in control of rebel headquarters in Slovyansk, a city of about 100,000 that has been a center of the fighting between Kiev’s troops and the pro-Russian insurgents.

Soldiers raised the Ukrainian flag over the City Council building, while troops carried out stockpiles of weapons from the city’s administrative and police buildings, which have been under rebel control since early April.

“It’s not a total victory. But the purging of Slovyansk of these bands, made up of people armed to the teeth, has incredible symbolic importance,” Poroshenko said in a statement posted on his official website.

Ukraine’s newly appointed Minister of Defense, Valery Heletey, was milling around with troops in the city center. He said that three planes with food and other supplies will soon arrive in Slovyansk.

A spokesman for the National Security and Defense Council said that mopping-up operations were continuing.

Andrei Purgin of the separatist Donetsk People’s Republic said that rebels were evacuating, but he claimed the army’s campaign had left the city “in ruins.”

The capture of Slovyansk represented the government’s biggest victory since it abandoned a shaky cease-fire last week and launched an offensive against the separatists.

Until now, the Ukrainian army had often appeared feckless in the months-long campaign against the rebels. On Thursday, Poroshenko shook up his defense team, appointing Ukraine’s third defense minister since the former president was ousted in February.

It was not yet clear whether the latest advance has permanently crippled the rebels, many of whom are moving to other cities.

In the city of Donetsk, streets were deserted Saturday as local officials urged people to stay at home. They said a battle was ongoing near the Donetsk city airport.

“Militants from Slovyansk and Kramatorsk have arrived in Donetsk,” said Maxim Rovinsky, spokesman for the City Council.

Some rebels played down the significance of Ukraine’s advances. Pavel Gubarev, the self-described governor of the Donetsk People’s Republic, wrote online that the rebels had staged a tactical retreat.

Others in the rebels’ ranks pleaded publicly with Russia to assist the rebels.

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